List of un-assigned keys in Blender 2.69
I don't think it can be nice and practical to list all un-assigned keys, since there obviously are too many posibilities with combinations. Furthermore, same keys can be used in different modes and contexts without conflicts, which extends the possibilities even further.

However, as gandalf3 suggested, you can view all shortcuts in User Preferences > Input. Also, when switching Filter Type to Key Binding, it allows you to search specific key that is currently assigned:



If nothing is filtered, then congratulations, it hasn't yet been assigned to any operators among all contexts. You can use it 100% safely: